In recent years, there has been a growing need for using radio waves in a ubiquitous network society and thus, a millimeter-wave-band wireless system, such as a wireless personal area network (WPAN) to provide a home wireless broadband service or a millimeter-wave radar for supporting safe and secure driving, has begun to be used. In addition, a 100-GHz ultra wideband wireless system has been actively developed.
In the second-order harmonic evaluation of a wireless system in a frequency band of 60 GHz to 70 GHz or the evaluation of a wireless signal in an ultra-wide frequency band of 100 GHz, as the frequency increases, the noise level of a measurement device and the conversion loss of a mixer increase and thus, the frequency accuracy is reduced. Therefore, a technique for measuring a wireless signal with a frequency higher than 100 GHz with high sensitivity and high accuracy has not been established. In addition, in the measurement technique according to the related art, it is difficult to separate harmonics of a local oscillation signal from the measurement result and to strictly measure, for example, unnecessary radiation.
Various circuit techniques including a narrow-band filter, such as a millimeter-wave-band filter for suppressing an image response and a high-order harmonic response, need to be developed in order to overcome the aforementioned technical problems and to measure a wireless signal in an ultra wideband of 100 GHz with high sensitivity and high accuracy.
For example, as a frequency-variable filter used in the millimeter wave band, the following filters have been known: (a) a filter using a YIG resonator; (b) a filter in which a varactor diode is attached to a resonator; and (c) a Fabry-Perot resonator.
As the filter (a) using the YIG resonator, a filter which can use a frequency up to about 80 GHz has been known. As the filter (b) in which the varactor diode is attached to the resonator, a filter which can use a frequency of up to about 40 GHz has been known. However, it is difficult to manufacture the filter with a frequency higher than 100 GHz.
In contrast, the Fabry-Perot resonator (c) has been used often in the optical field and Non-Patent Document 1 discloses a technique which uses the Fabry-Perot resonator (c) for millimeter waves. Non-Patent Document 1 discloses a confocal Fabry-Perot resonator in which a pair of spherical reflecting mirrors that reflect millimeter waves are arranged so as to be opposite to each other, with a gap equal to a curvature radius therebetween, to obtain a large Q value.